This invention relates to a method and a device for restoration of lakes by oxygenation of the water. More particularly the invention intends to solve the problem of air-treatment of lakes the depth of which exceeds 8 to 10 meters without disturbing of the thermal stratification of the water.
During the summer, the water of such lakes is stratified into two different layers, one upper warmer layer, epilimnion, and one lower colder layer, hypolimnion. The border between the epilimnion and the hypolimnion layer is normally situated at a depth of 8 to 10 meters. In very large lakes the hypolimnion may be located at a depth ranging between 30 and 50 meters. The surface layer, epilimnion, has intimate contact with the atmosphere and is therefore able to take up some oxygen. In this upper layer, plankton algae produce organic matter with oxygen as a by-product. When nutrient concentrations in the upper layer are high by nature, or by action of man, a rapid production of organic matter takes place. A large proportion of this matter settles into the lower colder layer where it is broken down by bacteria into its inorganic components. However, these processes will only occur if there is oxygen in the hypolimnion water.
If there is insufficient oxygen for this organic break-down, nutrient salts will diffuse into the water from the sediment layer at the bottom of the lake during the stagnant summer period. In the course of the following spring circulation, these nutrient salts are distributed throughout the bulk of water and thus become available for further organic production. This continuous increase in production of organic matter makes the oxygen balance progressively worse and the lake cannot avoid this development without outside help.
One way to help the lake out of this situation is to supply oxygen to the water. The oxygen must be supplied to the lower, oxygen-consuming and oxygen-deficient layer, hypolimnion. In this connection it becomes important not to mix the hypolimnion water with the oxygen-rich surface water because a total oxygen deficiency in the lake could be the result.
According to one previously proposed method for oxygenation of lake hypolimnion, water is transported to the surface by means of an air-lift pump and, after having been in contact with the atmosphere, it is returned to its original depth.
This known method has the disadvantage that it requires very bulky equipment which is difficult to handle. The air-lift pump has to be at least 10 meters high to penetrate the epilimnion layer.
Another disadvantage inherent in this method is that the water is brought into contact with air under atmospheric pressure with consequent relatively low oxygen solubility.
According to this invention these problems are solved by air-treatment of the water within the hypolimnion itself, which means that oxygen is introduced into the water at the pressure existing in the hypolimnion. If, for instance, the water is oxygenated at a depth of twenty meters, the amount of oxygen soluble in water is about three times higher than the corresponding amount at the surface. It also means that the device required is much smaller than the previously described air-lift pump. A device for air-treatment of water within the hypolimnion is included in the invention.